This invention relates generally to contactless length measurement devices, and, more particularly to a test sample support assembly for use with such contactless length measurement devices.
Dimensionally stable materials find great utility as aerospace components such as microwave filters and waveguides, antenna structures and supports, laser or optic platforms, instrument parts, solar cell connectors and mounts, cryogenic piping and the like. In addition, low expansion materials are required for guidance systems, space telescopes, and most communications, navigation, scientific and surveillance satlilites.
Unfortunately, in spite of "near-zero" expansivities of such dimensionally stable materials as Invar, TiO.sub.2 -SiO.sub.2 glasses, and multi-ply graphite-epoxy composites, no materials yet exist that exhibit thermal strain of less than 6.times.10.sup.-5 over the aerospace working range, 0.degree..+-.200.degree. C. Consequently, a high-precision dilatometer is needed to measure dimensional changes over a wide temperature range. Ideally, effects that are due to residual stress relief, moisture desorption, and thermal cycling, as well as expansivity, should be measurable for both aerospace components and specially fabricated test samples.
It is well recognized that the utilization of remote or "contactless" measurement devices are the most effective in overcoming the above mentioned problems. This is so, since the thermal expansion coefficient, by definition, must be characterized at a constant pressure: EQU .beta.=V.sup.-1 (dV/dT).sub.p or .alpha.=( d.epsilon./dT).sub.p
Since expansivity varies with applied stress and many materials lack a true elastic limit, mechanical constraints should be avoided. Contacts may cause microcreep and surface contamination or damage. The position, stability, and thermal properties of contacts affect measurements, especially of real time data, because of thermal lag. A contactless measurement technique permits arbitrary sample size or shape, thereby minimizing fabrication effects on a sample or component. In addition, contactless measurements reduce temperature range restrictions and equilibration requirements and permit simultaneous thermal diffusivity measurements.
Contactless length measurement techniques have been performed by a variety of apparatus. Stationary light beams, from lasers or autocollimators, may be reflected off sample ends. The Fototonic fiber optics approach is, in principal, similar. Scanning techniques include single laser systems, multiple lasers or multiple sensors. Photographic techniques include Moire, speckle, and holographic interferometry. The most accurate approach, however, has been accomplished by Michelson interferometry.
The use of such interferometric techniques for length measurements results in the elimination of dependence on a reference material. The laser frequency can be readily known and stabilized, through use of the Lamb dip, to one part in 10.sup.9 in 500 hours. When all the optics are placed in a test (vacuum) chamber possible errors from variable beam speeds, window effects, or operators are minimized.
With the basic Michelson interferometer measurement determining device, one arm of the interferometer includes both ends of the test sample, which unfortunately, results in a large optical path length difference (OPLD) between the two beams required to recombine in order to form the necessary fringe pattern. It has been determined that these several possible sources of error are inherent in such an approach. Because the optical path length to the sample was substantially greater (70 times) than that to the reference mirror, errors arose as a result of difference in pressure or temperature of the residual gas in the two optical path lengths (OPLs).
This situation has been overcome by the utilization of the Two Channel Michelson Interferometer. Unfortunately, since all the optics utilized even in the Two Channel Michelson Interferometer used in the sample optical path length were held on the same support plate, any temperature changes in any part of this plate would change the sample optical path length. Consequently, the interferometer would confuse this optical path length change with a sample length change.
In principle, such an error could be avoided by a zero coefficient of thermal expansion support plate. This would be approximated by ultra-low-expansion (ULE) glass near room temperature (CTE .about.0.+-.0.03.times.10.sup.-6 degrees C..sup.-1). A sufficiently large and stiff plate, however, is extremely expensive (stiffness provides immunity from vacuum chamber distortions on pump down or ambient temperature fluctuations). The error might also be avoided by the use of a water-cooled copper base plate attached to a thermostatically controlled bath. Undesirable vibrations could result, however, because the optics would have to be attached rigidly to the plate.
It is therefore clearly apparent that a need arises for a support assembly which is capable of supporting a test sample or the like within a length measuring device and yet remain unaffected by the surrounding temperatures. In so doing sample optical path length would not be altered due to the surrounding temperatures.